A wind turbine is preferably controlled to extract as much energy as possible from the wind. Therefore, most wind turbines feeding into a power grid have pitch control systems that can control the rotor blade pitch angles in response to wind speed or grid requirements, and also to furl the blades in the event of excessively high winds. The pitch control system of a wind turbine is generally located in the hub. Pitch control systems have conventionally been realized as hydraulic systems. However, electric pitch systems are becoming more widespread for various reasons such as ease of maintenance and for environmental considerations.
Various embodiments are possible for a pitch-controlled rotor blade. In each case, the rotor blade is caused to turn about its longitudinal axis. In one realisation, a circular blade root end is mounted to a circular pitch ring, which in turn is mounted to the hub of the wind turbine. The pitch ring can have an inner or outer toothed ring which is driven by a corresponding pinion or gear of the pitch system. A large annular bearing such as a fluid bearing is required to ensure smooth motion of the blade relative to the hub. This type of blade design is associated with very high loading on the pitch system.
In an alternative design, the blade is mounted on a cylindrical blade shaft, which is rigidly secured to the hub and which extends radially outward from the hub. Two or more annular bearings arranged between the blade shaft and the blade, separated by a certain bearing separation distance, ensure a smooth motion of the blade as it rotates about its longitudinal axis. The length of the blade shaft can comprise more than 15% of the total blade length, so that loads acting on the blade are evenly transferred to the bearings on the blade shaft. In this case also, the blade pitch angle can be controlled by an annular gear comprising a toothed ring about the blade root end, arranged to engage with a pinion that is driven by a pitch system. A problem associated with this type of design is that components of the pitch system—for example a drive unit containing the transmission systems, cables connecting the drive units to a power source and to a controller, etc.—must be accommodated in the hub area close to the blades. Furthermore, the pitch drive unit of each rotor blade must be mounted so that it is stationary relative to the blade shaft. In a known embodiment, the pitch drive unit is mounted to an exposed inner portion of the blade shaft between the hub and the root end rotor blade. In this design, the blade shaft length comprises the bearing separation distance and this “extra length”, since the exposed portion of the blade shaft must be long enough to accommodate the pitch drive unit. However, since the rotor blade and the bearings must then be located further outward from the hub, the loading experienced by the bearings is correspondingly high. The bearings must be dimensioned to withstand this loading. This adds to the cost of the bearings. Alternatively or in addition, the blade shaft must be made longer. The necessity of having to reserve a length of the blade shaft to mount the pitch drive unit therefore adds to the overall cost and weight, particularly since the blade shaft is usually made of cast steel.